adb: Another attempted workaround for the adb disconnect problem.

For adb shell commands (as well as shell based commands like logcat and bugreport) we now
batch USB writes into 4K chunks instead of sending small packets of data as they come in.
A timeout prevents us from blocking for more than 100ms when our 4K buffer is partially full.

Signed-off-by: Mike Lockwood <lockwood@android.com>